Disease is simply known as a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not due to any immediate external injury. Diseases are often known to be medical conditions that are associated with specific symptoms and signs.

A disease can be caused by external factors like pathogens or by internal dysfunctions. Under internal dysfunctions of the immune system, variety of different diseases can occur, such as various forms of, allergies and autoimmune, immunodeficiency, disorders, hypersensitivity, etc.

In humans, disease is often used more broadly to refer to any condition that causes pain, dysfunction, distress, social problems, or death to the person afflicted, or similar problems for those in contact with the person. In this case, it sometimes includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviors, and a typical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories. Diseases can affect people not only physically, but also mentally, as contracting and living with a disease can alter the affected person’s perspective on life.

However, death due to disease is called death by natural causes. There are four main types of disease: infectious diseases, deficiency diseases, hereditary diseases (including both genetic diseases and non-genetic hereditary diseases), and physiological diseases.

In addition, diseases can be classified in some ways, like communicable versus non-communicable diseases of which the deadliest diseases in humans are coronary artery disease (blood flow obstruction), then by cerebrovascular disease and lower respiratory infections. In developed countries, the diseases that cause the most sickness overall are neuropsychiatric conditions, such as depression and anxiety.